Hydrogen-occluding alloy and method for production thereof

ABSTRACT

A hydrogen storage alloy and its production method are disclosed, which has an extremely high effective hydrogen storage capacity in the pressure range from 0.001 to 10 MPa, and a variety of use. The alloy is principally of a body-centered cubic crystal structure, and represented by the compositional formula Cr a Ti b V c Fe d M e X f  (M: Al etc.; X: La etc.; 30≦a≦70, 20≦b≦50, 5≦c≦20, 0≦d≦10, 0≦e≦10, and 0≦f≦10, a+b+c+d+e+f=100). The alloy contains 0.005 to 0.150 wt % of O 2 , and has hydrogen absorption-desorption capability of not less than 2.2% of its weight from 0 to 100° C. and from 0.001 to 10 MPa. The method includes step (a) of melting starting materials for the alloy, deoxidizing step (b) such as step (b1) of blowing Ar into the alloy melt, and casting step (c).

FIELD OF ART

[0001] The present invention relates to hydrogen storage alloys that arecapable of absorbing and desorbing hydrogen in a temperature range fromroom temperature to 100° C., and methods for producing the same. Inparticular, the present invention relates to hydrogen storage alloysuseful for vehicle on-board or stationary hydrogen storage, and methodsfor producing the same.

BACKGROUND ART

[0002] Hydrogen, which reacts with oxygen to generate water withoutgenerating toxic substances, has been attracting attention as cleanenergy. Handling of hydrogen, however, is delicate due to its explosivereactivity with oxygen at a certain ratio. Storage alloys that absorband store hydrogen in metals, have been noticed as being capable ofsafely storing more hydrogen than hydrogen cylinders.

[0003] Hydrogen storage alloys have recently been in use for productionof anodes of rechargeable batteries, and their production has beensharply increasing. For keeping up with tightening of the vehicleemissions limit from the year 2004, leading vehicle manufacturers havebeen developing electric vehicles equipped with rechargeable batteriesor proton-exchange membrane fuel cells, in which electricity isgenerated by methanol reforming to generate hydrogen followed byreaction with atmospheric oxygen. On such electric vehicles, hydrogencylinders or hydrogen storage alloys are installed for supplyinghydrogen in initial start-up and for compensation for load fluctuation.

[0004] Hybrid cars, which are equipped with a gasoline engine and amotor, are now on the market. Such hybrid cars employ AB₅-type hydrogenstorage alloys. For extending the travel distance per charge andlightening the vehicle body weight, improvement and development ofalloys having a higher hydrogen storage capacity are strongly demanded.

[0005] The AB₅-type hydrogen storage alloys now in general use havehydrogen storage capacities of about 1.4% of the alloy weight. Ashydrogen storage alloys having higher hydrogen storage capacities thanthose of the AB₅-type hydrogen storage alloys, Fe—Ti based alloys areconventionally known. The Fe—Ti based alloys are advantageous inrelatively low price and the plateau pressure of 0.4 to 0.6 MPa at roomtemperature, but disadvantageous in hardness of activation. However,these alloys have hydrogen storage capacities of as high as 1.7% of thealloy weight, which is quite promising.

[0006] MgNi₂ alloys are known to have a high hydrogen storage capacity.However, the operational temperature of these alloys is as high as 300°C., which is too high for general household use or home electricappliances.

[0007] As hydrogen storage alloys usable in the temperature range fromroom temperature to 100° C., those having a body-centered cubicstructure (referred to as BCC hereinbelow) have been receivingattention. In the BCC structure, there is a space in the center of atetrahedral or octahedral structure, in which space hydrogen is stored.The BCC alloys have been reported to have a theoretical hydrogen storagecapacity of 4.0% of the alloy weight.

[0008] As BCC hydrogen storage alloys, JP-10-110225-A discloses ahydrogen absorbing alloy having a composition expressed by the generalformula Ti_(x)Cr_(y)V_(z) (x+y+z=100) wherein a body-centered cubicstructure appears and a spinodal decomposition occurs with the exceptionof a Laves phase, and the structure has a regular periodical structureformed by the spinodal decomposition, and its apparent lattice constantis at least 0.2950 nm but is not greater than 0.3060 nm. JP-10-310833-Adiscloses a Ti—V—Cr based hydrogen storage alloy. JP-10-121180-Adiscloses an alloy of the formula Ti_((100-a-b))—Cr_(a)—X_(b) (40<a<70,0<b<20) having the BCC structure and containing Mo or W. JP-11-106859-Adiscloses a Ti—V—Cr based alloy to which one or more quaternary elementsselected from the group consisting of Mn, Co, Ni, Zr, Nb, Hf, Ta, and Alare added, wherein the ratio of the components in atomic % is 14<Ti<60,14<Cr<60, 9<V<60, 0<quaternary element <8 in total of 100%, and themetal structure is the BCC structure to improve the flatness of theplateau. The above-mentioned alloys have the BCC structure, but have thehydrogen storage capacities of only less than 2.5%.

[0009] Regarding hydrogen storage alloys having the BCC structure andcontaining Fe, JP-9-49034-A discloses a method for producing, from aFe—V alloy as a starting material, a hydrogen storage alloy having theBCC structure composed of not less than three elements including atleast V and Fe. The hydrogen storage capacity of the alloy obtained bythis method, however, is below 2.5%. Japanese Patent No. 2743123discloses a Ti—Cr—V—Fe hydrogen storage alloy, but this alloy also hasthe hydrogen storage capacity of not higher than 2.5%.

[0010] It has been reported that the storage capacity of hydrogenstorage alloys is influenced by the oxygen content in the alloys (J.Alloys Comp. 265 (1998), p257-263). In the text of Special PublicSymposium '99 (Dec. 17, 1999) of MH Riyou Kaihatsu Kenkyu Kai, it isreported that the oxygen concentration in the alloy was decreased from1% to 0.06% by alloying a crude material produced by thermit process asa basic material containing 14 atom % of V, 1 atom % of Ni, and Nb, andother component elements and 5 atom % of misch metal (abbreviated as Mmhereinbelow) by arc melting under a reduced pressure argon atmosphere,to thereby succeed in remarkable improvement in the hydrogen storagecapacity. However, the hydrogen storage capacity of the alloy system isstill less than 2.0%.

[0011] Conventionally, performance of hydrogen storage alloys has beenevaluated based on the maximum hydrogen storage capacity in repeatedhydrogen absorption and desorption at a certain temperature, or on thehydrogen storage capacity measured from the origin taken under vacuum.However, in practical use of hydrogen storage alloys in fuel cells, whatis important is not the maximum hydrogen storage capacity, but theamount of hydrogen that is involved in absorption and desorption in thepressure range from 0.001 to 10 MPa, that is, the available hydrogen(referred to as effective hydrogen storage capacity hereinbelow).

[0012] BCC alloys typically have two plateaus, and the maximum hydrogenstorage capacity or the storage capacity in the first cycle of the BCCalloys containing V conventionally includes the hydrogen content in thefirst plateau appearing at lower pressure, which is actually notavailable for use. Thus the measured amount is far from the effectivehydrogen storage capacity. Also in the conventional measurement from theorigin taken under vacuum, the hydrogen content in an impracticallylower pressure range is included in the measured amount, so that theamount thus measured is larger than the effective hydrogen storagecapacity.

[0013] In sum, the hydrogen storage capacities of the BCC hydrogenstorage alloys developed to date have been reported to exceed 2.5%, butthese are evaluated in terms of the maximum hydrogen storage capacity,not in terms of the effective hydrogen storage capacity. Thus there isconventionally known no alloy containing not more than 20 atom % of Vthat has a hydrogen storage capacity exceeding 2.2% in terms of theeffective hydrogen storage capacity in the pressure range from 0.001 to10 MPa and the temperature range from room temperature to 100° C.

[0014] The BCC hydrogen storage alloys are produced by rapid quenchingfrom the high temperature BCC region, in order to give the BCC structureto the alloy in the service temperature range. Thus alloys having a wideBCC region in the higher temperature range in the phase diagram areadvantageous for production of hydrogen storage alloys. For extendingthe BCC region in the higher temperature range, V is added to the alloycomposition, as typically in Ti—Cr—V based alloys, in which the BCCregion is extended in proportion to the V content. However, V has twodisadvantages in use as a principal component. One is the high cost of Vmetal. The higher the V content, the more expensive the resultinghydrogen storage alloy will be, and the use of the alloy may be limited.The other is the melting point of V, which is as high as 1910° C. Formelting V metal in the Ti—Cr—V based alloy, the furnace temperature israised, which causes reduction of the furnace refractory materials by aprincipal element Ti of the alloy. This shortens the life of therefractory materials of the melting furnace, and increases the oxygencontent of the resulting alloy. Thus in the production of Ti—Cr—V basedalloys, reduction in the amount of expensive V and lowering of themelting temperature are important factors.

[0015] Instead of V metal, inexpensive ferrovanadium (Fe—V) may be takeninto consideration as a material of hydrogen storage alloys. However,Fe—V has a very high oxygen content of 0.5 to 1.5%, which leads toincrease in the oxygen content and lowering of the hydrogen storageproperty of the resulting hydrogen storage alloy.

DISCLOSURE OF THE INVENTION

[0016] It is an object of the present invention to provide a hydrogenstorage alloy having an extremely high effective hydrogen storagecapacity in the pressure range from 0.001 to 10 MPa, and useful in avariety of use, and a method for producing such an alloy.

[0017] It is another object of the present invention to provide a methodfor producing a hydrogen storage alloy that enables, at a temperaturelower than the melting temperature of V, ready production of a hydrogenstorage alloy having an extremely high effective hydrogen storagecapacity in the pressure range from 0.001 to 10 MPa and useful in avariety of use.

[0018] According to the present invention, there is provided a hydrogenstorage alloy principally of a BCC crystal structure, and represented bythe compositional formula Cr_(a)Ti_(b)V_(c)Fe_(d)M_(e)X_(f), wherein Mstands for one or more elements selected from the group consisting ofAl, Mo, and W; X stands for one or more elements selected from the groupconsisting of La, Mm, Ca, and Mg; a, b, c, d, e, and f each denotes avalue in atomic percent, and 30≦a≦70, 20≦b≦50, 5≦c≦20, 0≦d≦10, 0≦e≦10,and 0≦f≦10, provided that a+b+c+d+e+f=100,

[0019] wherein said alloy comprises 0.005 to 0.150 wt % of O₂, and hashydrogen absorbing-desorbing capability of not less than 2.2% of itsweight in a temperature range from 0 to 100° C. and in a pressure rangefrom 0.001 to 10 MPa.

[0020] According to the present invention, there is also provided amethod for producing the hydrogen storage alloy mentioned above,comprising:

[0021] melting step (a) of melting starting materials for the hydrogenstorage alloy to prepare an alloy melt;

[0022] at least one deoxidizing step (b) selected from the groupconsisting of deoxidizing step (b1) of blowing argon gas into the alloymelt, deoxidizing step (b2) of retaining the alloy melt in the vacuum ofnot higher than 0.1 Pa, and deoxidizing step (b3) of adding andretaining in the alloy melt one or more elements selected from the groupconsisting of La, Mm, Ca, and Mg;

[0023] casting step (c) of solidifying the alloy melt; and optionally,

[0024] step (d) of retaining a solidified alloy in a temperature rangefrom 1150 to 1450° C. for 1 to 180 minutes, followed by cooling to 400°C. or lower at a cooling rate of not lower than 100° C./sec.

PREFERRED EMBODIMENTS OF THE INVENTION

[0025] The present invention will now be discussed in detail.

[0026] The hydrogen storage alloy according to the present invention isprincipally of a body-centered cubic structure. “Principally” as usedherein means that the alloy is of a body-centered cubic structure to theextent that no secondary phase different from BCC is clearly observedunder an X-ray diffractometer.

[0027] The hydrogen storage alloy of the present invention isrepresented by the compositional formulaCr_(a)Ti_(b)V_(c)Fe_(d)M_(e)X_(f), and contains a specific amount of O₂.In the formula, M stands for one or more elements selected from thegroup consisting of Al, Mo, and W, and X stands for one or more elementsselected from the group consisting of La, Mm, Ca, and Mg. a, b, c, d, e,and f each denotes a value in atomic percent, and 30≦a≦70, 20≦b≦50,5≦c≦20, 0≦d≦10, 0≦e≦10, and 0≦f≦10, provided that a+b+c+d+e+f=100.

[0028] In the above formula, Ti, Cr, and Fe are indispensable elementsfor rendering the crystal structure of the alloy BCC, and must becontained at the particular ratio mentioned above.

[0029] In the formula, V is an expensive material, and if the V contentexceeds 20 atom %, the price of the resulting hydrogen storage alloywill be too expensive to be marketed, whereas with the V content of lessthan 5 atom %, the BCC structure is hard to be obtained. If the Fecontent exceeds 10 atom %, the hydrogen storage capacity of the alloysharply drops. Thus, d representing the Fe content is preferably 1≦d≦10.

[0030] Among the elements of M in the formula, if the content of Alexceeds 10 atom %, the hydrogen storage capacity is adversely affected.Addition of up to 20 atom % of Mo or W to a Ti—Cr alloy renders thealloy BCC. However, for the Cr—Ti—V—Fe alloy of the present invention,in which a small amount of V and Fe are contained, if the content of Moand/or W exceeds 10 atom %, the alloy is not rendered BCC, and thehydrogen storage capacity is lowered.

[0031] The component X in the formula, which is one or more elementsselected from the group consisting of La, Mm, Ca, and Mg, is containedin the hydrogen storage alloy of the present invention when added as adeoxidizer during production of the alloy. X is usually added in anamount 1.5 times the amount of oxygen in the starting materials for thealloy, but if the resulting hydrogen storage alloy contains more than 10atom % of X, the effective hydrogen storage capacity is less than 2.2%.

[0032] With the hydrogen storage alloy of the present invention, even ifthe ratio of M and/or X in the formula is 0, the desired effectivehydrogen storage capacity may be achieved. When the present hydrogenstorage alloy contains M and/or X, in other words, when 0<e≦10 and/or0<f≦10 independently, e and f are preferably 1≦e≦10 and 1≦f≦10,independently. In other words, the hydrogen storage alloy of the presentinvention has three types, namely, those containing neither M or X,those containing either M or X, and those containing both M and X.

[0033] The present hydrogen storage alloy is represented by the abovecompositional formula, and contains O₂ in an amount of not less than0.005 wt % and not more than 0.150 wt %, preferably not less than 0.04wt % and not more than 0.100 wt %. With the O₂ content exceeding 0.150wt %, the desired effective hydrogen storage capacity is hard to beobtained. On the other hand, the alloy containing less than 0.005 wt %of O₂ is hard to be produced.

[0034] The hydrogen storage alloy of the present invention may containinevitable components in addition to the above components, as long asthe desired objects of the present invention are not impaired.

[0035] The present hydrogen storage alloy has the hydrogenabsorption-desorption capability of not less than 2.2%, preferably notless than 2.4% of its weight in the temperature range from 0 to 100° C.and in the pressure range from 0.001 to 10 MPa. The upper limit of thehydrogen absorption-desorption capability is not particularly imposed,but is about 3.0%.

[0036] The present hydrogen storage alloy may preferably be produced bya method according to the present invention essentially including steps(a) to (c), and optionally step (d).

[0037] That is, the method of the present invention includes the meltingstep (a) of melting the starting materials for the present hydrogenstorage alloy to prepare an alloy melt, at least one deoxidizing step(b) selected from the group consisting of deoxidizing step (b1) ofblowing argon gas into the alloy melt, deoxidizing step (b2) ofretaining the alloy melt in the vacuum of not higher than 0.1 Pa, anddeoxidizing step (b3) of adding and retaining in the alloy melt one ormore elements selected from the group consisting of La, Mm, Ca, and Mg,and casting step (c) of solidifying the alloy melt, and optionally step(d) of retaining the solidified alloy in a temperature range from 1150to 1450° C. for 1 to 180 minutes, followed by cooling to 400° C. orlower at a cooling rate of not lower than 100° C./sec.

[0038] The starting materials for the hydrogen storage alloy in step (a)contain Cr, Ti, V, and Fe, and optionally component M that is one ormore elements selected from the group consisting of Al, Mo, and W,and/or component X that is one or more elements selected from the groupconsisting of La, Mm, Ca, and Mg. The mixing ratio of the respectivecomponents may suitably be selected so that the resulting alloy has thedesired composition as mentioned above.

[0039] Each of the starting materials may either be an unalloyed metalor an alloy. As an alloy, for example, a Fe—V, Cr—Ti, or Cr—V alloyhaving a lower melting point than that of V metal, may be used. Vprepared by the thermit process for reducing the oxygen content in Vmetal may be used. In this case, since V prepared by this processusually contains Al, the amount of this residual Al must be included inthe ratio defined in the desired composition mentioned above. Each ofthe starting materials may be melted in any order, either simultaneouslyor in several separate batches. Some of the starting materials may evenbe melted during the deoxidizing step (b) to be discussed later.

[0040] The starting materials for the alloy may be melted, for example,by arc melting or in a high frequency furnace. The melting is preferablyperformed in an argon atmosphere. The temperature for melting is notlower than the melting temperature of the starting materials, and theupper limit may preferably be 1700° C. For lowering the meltingtemperature, a Fe—V alloy, which has a lower melting point than that ofV metal, may preferably be used. This Fe—V alloy has a high oxygencontent, which causes lower hydrogen absorption-desorption capability,and is thus not suitable for producing alloys having high hydrogenabsorption-desorption capability. According to the present invention,however, such an alloy may still be utilized as a starting material,since the present method includes the step for reducing the oxygencontent of the objective alloy.

[0041] Step (b) includes at least one deoxidizing step selected from thegroup consisting of steps (b1), (b2), and (b3), and two or more of thesesteps may be performed.

[0042] In the deoxidizing step (b1), argon gas is blown into the alloymelt prepared instep (a) for deoxidization. For effective deoxidization,the argon gas is blown into the alloy melt preferably for longer than 10seconds and shorter than 5 minutes. The amount of the argon gas to beblown may suitably be decided depending on the volume and amount of thealloy melt.

[0043] In the deoxidizing step (b2), the alloy melt prepared in step (a)is retained in the vacuum of not higher than 0.1 Pa for deoxidization.In the vacuum of higher than 0.1 Pa, efficient deoxidization may not beperformed. The duration of the deoxidization may preferably be 1 to 5minutes, but in view of the reactivity between the alloy melt and thecrucible, the duration is preferably kept minimum.

[0044] In the deoxidizing step (b3), one or more elements selected fromthe group consisting of La, Mm, Ca, and Mg are added and retained in thealloy melt. When the starting materials for the alloy in step (a)contain one or more elements selected from the group consisting of La,Mm, Ca, and Mg, step (b3) may be performed simply by retaining themolten starting materials for a time period for allowing deoxidization,preferably for 1 to 5 minutes. Step (b3) may alternatively be performed,after the alloy melt is prepared, by adding to the alloy melt thedesired amount of one or more elements selected from the groupconsisting of La, Mm, Ca, and Mg as a deoxidizer, melting, and retainingthe resulting alloy melt for the desired period of time mentioned above.Here, the La, Mm, Ca, Mg, or mixtures thereof added as a deoxidizer maybe or may not be included in the composition of the resulting alloy.When the resulting alloy does not contain these components, the alloyhas a composition of the formula wherein the ratio of X is 0. When theresulting alloy contains these components, their amounts have to beadjusted to be in the range for X.

[0045] When step (b3) wherein the deoxidizer is added afterwards andmelted is employed, it is preferred to perform this step after step (b1)and/or (b2) for effective reaction of the deoxidizer.

[0046] In the casting step (c), the alloy melt is solidified, which maybe performed by a conventional casting method such as metal mold castingor strip casting. The cooling conditions may suitably be selected, butfor their ready control, the strip casting may be preferred, which isalso favorable to production of readily pulverizable alloy strips of 2mm thick or less. The cooling conditions may be set, for example, togenerate the BCC structure in the higher temperature range bycontrolling the cooling rate. However, such conditions are not necessarywhen step (d) to be discussed later is employed, and the cooling ratemay be set at a lower rate.

[0047] When the optional step (d) is performed after the casting step(c), the alloy obtained in step (c) may be subjected to step (d) eitherin the as-cast form, or after suitable pulverization, homogenizing heattreatment, or aging heat treatment. When step (c) is followed by step(d), the cast alloy obtained in step (c) does not necessarily have theBCC structure, which may be generated in the following step (d).

[0048] In step (d), the alloy prepared in step (c) in the as-cast formor optionally after pulverization or various heat treatments, isretained in a temperature range from 1150 to 1450° C. for 1 to 180minutes, preferably in a temperature range from 1200 to 1400° C. for 5to 20 minutes, and then cooled to 400° C. or lower, preferably to aroundroom temperature, at a cooling rate of not lower than 100° C./sec,preferably 500 to 1000° C./sec. Step (d) is particularly needed forgenerating the BCC structure desired of the present hydrogen storagealloy when the BCC structure is not obtained under the solidifyingconditions in step (c).

[0049] The method of the present invention may optionally include othersteps in addition to the above, as long as the objects of the presentinvention are not impaired.

[0050] The hydrogen storage alloy of the present invention is of theparticular composition, principally of the BCC structure, and has theparticular O₂ content, which results in the high effective hydrogenstorage capacity that has never been achieved by conventional alloys.Thus, the present alloy is extremely useful for the vehicle on-boardpurpose such as for electric vehicles and hybrid cars, as well as forthe stationary hydrogen storage purpose. The method of the presentinvention uses the particular starting materials for the alloy andincludes the deoxidizing step (b) and optionally step (d) of particularheat treatment and cooling. Thus the hydrogen storage alloy of thepresent invention that is useful in various purposes, may easily beproduced at a temperature lower than the melting point of V.

EXAMPLES

[0051] The present invention will now be explained with reference toExamples and Comparative Examples, which do not intend to limit thepresent invention.

Examples 1-8 and Comparative Examples 1-2

[0052] Using V with 0.55 wt % oxygen content produced by the thermitprocess, a Cr—Ti—V—Fe or Cr—Ti—V—Fe—Al alloy was prepared. The obtainedalloy was used as a basic component, and La, Mm, Ca, or Mg was measuredout for preparing the objective composition shown in Table 1. The alloyand the measured metal in total of 20 g were each placed in awater-cooled copper mold, arc-melted in an argon atmosphere, turned upside down in the mold, and further melted. This operation was repeatedthree times, and La, Mm, Ca, or Mg was added to the alloy melt andretained, which is step (b3), to obtain a cast alloy.

[0053] 3 g of the cast alloy was measured out and placed in a PCTmeasuring system (PCT-4SWIN manufactured by SUZUKI SHOKAN CO., LTD.).Hydrogen absorption and desorption were repeatedly performed at 40° C.under the hydrogen pressure of 0.01 to 5 MPa, and the effective hydrogenstorage capacity was determined from the obtained PCT curve in the thirdcycle. The results are shown in Table 1.

[0054] Next, the obtained alloy was retained at 1400° C. for 10 minutes,cooled down to 300° C. at a cooling rate of 550 to 1000° C./sec, andfurther allowed to cool to room temperature. The composition of theresulting alloy was measured by infrared absorption with respect to theoxygen content, and by ICP atomic emission spectrochemical analysis withrespect to the other elements. Further, 3 g of the obtained alloy wasmeasured out and placed in a PCT measuring system (PCT-4SWINmanufactured by SUZUKI SHOKAN CO., LTD.). Hydrogen absorption anddesorption were repeatedly performed at 40° C. under the hydrogenpressure of 0.01 to 5 MPa, and the effective hydrogen storage capacitywas determined from the obtained PCT curve in the third cycle. The ratioof the BCC phase in the alloy was measured by X-ray diffraction. Theresults are shown in Table 1.

[0055] It is seen from Table 1 that the alloys of the present inventionexhibited the effective hydrogen storage capacities of not less than2.2% even when the as-cast alloy had a lower capacity. On the otherhand, all of the alloys of the Comparative Examples having conventionalcompositions exhibited the effective hydrogen storage capacities of lessthan 2.2%. TABLE 1 Alloy Composition (atomic %) As-cast AlloyHeat-treated Alloy Basic Effective Effective Composition of Ratio ofHydrogen Ratio of Hydrogen Alloy Starting O₂ Content BOC Phase StorageBOC Phase Storage Material La Mm Ca Mg (wt %) (%) Capacity (%) (%)Capacity (%) Example 1 Cr₄₆Ti₂₉V₁₃Fe₄A₁₃  5 — — — 0.041 100 2.42 1002.70 Example 2 — 5 — — 0.043 100 2.35 100 2.65 Example 3 — — 5 — 0.055100 2.32 100 2.55 Example 4 — — — 5 0.052 100 2.12 100 2.53 Example 5Cr₄₈Ti₃₂V₁₃Fe  6 — — — 0.042 100 2.43 100 2.70 Example 6 Cr₅₀Ti₃₃V₁₅Fe 1 — — — 0.042 100 2.43 100 2.70 Example 7 Cr₅₅Ti₃₄V₆FeMo₂  2 — — —0.078 10 1.35 100 2.70 Example 8 Cr₅₄Ti₃₄V₆Fe₂Al  3 — — — 0.065 10 1.25100 2.68 Comparative Cr₅₃Ti₂₇V₁₃Fe₄Al₃ — — — — 0.160 100 1.70 100 2.10Example 1 Comparative Cr₄₃Ti₂₈V₁₃Fe₂Al₂ 12 — — — 0.044 100 1.80 100 2.15Example 2

Examples 9-15 and Comparative Examples 3-4

[0056] Fe—V with 0.55 wt % oxygen content produced by the thermitprocess and a Cr—Ti alloy were initially placed in a MgO crucible,melted at 1650° C., and retained under the vacuum of 0.08 MPa for 3minutes. Then the atmosphere was replaced with argon, and pure argon wasblown into the alloy melt by means of a lance. The alloy melt wasretained under the vacuum of 0.08 MPa for 3 minutes again, the alloycomposition was precisely adjusted, and La, Mm, Ca, or Mg was added.When reached 1680° C., the alloy melt was poured onto a copper rollrotating at 1 m/sec or 15 m/sec to produce alloy strips by stripcasting. 3 g of the cast alloy was measured out and placed in a PCTmeasuring system (PCT-4SWIN manufactured by SUZUKI SHOKAN CO., LTD.).Hydrogen absorption and desorption were repeatedly performed at 40° C.under the hydrogen pressure of 0.01 to 5 MPa, and the effective hydrogenstorage capacity was determined from the obtained PCT curve in the thirdcycle. The results are shown in Table 2.

[0057] Next, the obtained alloy strips were retained at 1400° C. for 10minutes, and water-cooled to room temperature at a cooling rate of 1000°C./sec. The composition of the basic alloy, amount of La, Mm, Ca, or Mg,and O₂ content of each alloy were measured in the same way as inExamples 1 to 8. Further, 3 g of the obtained alloy was measured out andplaced in a PCT measuring system (PCT-4SWIN manufactured by SUZUKISHOKAN CO., LTD.). Hydrogen absorption and desorption were repeatedlyperformed at 40° C. under the hydrogen pressure of 0.01 to 5 MPa, andthe effective hydrogen storage capacity was determined from the obtainedPCT curve in the third cycle. The results are shown in Table 2.

[0058] It is seen from Table 2 that all of the alloys produced inExamples had the oxygen content of less than 0.1 wt %. Even though theeffective hydrogen storage capacities of some of the as-cast alloys ofthe present invention determined from the PCT curve were less than 2.2%,the subsequent heat treatment improved the effective hydrogen storagecapacities to over 2.2% in all the alloys of the present invention.TABLE 2 Alloy Composition (atomic %) As-cast Alloy Heat-treated AlloyBasic Effective Effective Composition of Ratio of Hydrogen Ratio ofHydrogen Alloy Starting O₂ Content BOC Phase Storage BOC Phase StorageMaterial La Mm Ca Mg (wt %) (%) Capacity (%) (%) Capacity (%) Example 9Cr₄₆Ti₂₉V₁₄Fe₃Al₃  5 — — — 0.053 100 2.40 100 2.60 Example 10 — 5 — —0.061 100 2.20 100 2.43 Example 11 — — 5 — 0.077 100 2.20 100 2.45Example 12 — — — 5 0.062 100 2.10 100 2.41 Example 13 Cr₄₈Ti₃₂V₁₅Fe  4 —— — 0.052 100 2.40 100 2.62 Example 14 Cr₅₅Ti₃₄V₇FeMo  2 — — — 0.075 201.40 100 2.60 Example 15 Cr₅₄Ti₃₄V₇Fe₂  3 — — — 0.065 20 1.25 100 2.58Comparative Cr₅₃Ti₂₇V₁₄Fe₃Al₃ — — — — 0.180 100 1.80 100 2.15 Example 3Comparative Cr₄₃Ti₂₈V₁₃Fe₂Al₂ 12 — — — 0.054 100 1.70 100 2.10 Example 4

Comparative Example 5

[0059] A Cr₄₉Ti₃₁V₁₅FeLa₄ alloy was prepared by melting in a highfrequency furnace a Fe—V alloy with 0.65 wt % oxygen content prepared bythe thermit process, a Cr—V alloy, and metal Ti as main staringmaterials. Specifically, these starting materials were initially placedin a MgO crucible, melted at 1650° C., and poured onto a copper rollrotating at im/sec to produce alloy strips. Then the obtained alloystrips were retained at 1300° C. for 10 minutes, and rapidly quenched toroom temperature, to thereby obtain an objective alloy. The obtainedalloy was subjected to the measurement of the oxygen content and thedetermination of the PCT curve in the same way as in Examples 1-8. Theresults are shown in Table 3.

Example 16

[0060] The same starting materials as in Comparative Example 5 weremelted at 1650° C., and retained under the vacuum of 0.06 MPa for 5minutes. Then the atmosphere was replaced with argon, and pure argon wasblown into the alloy melt by means of a lance. The alloy melt wasretained under the vacuum of 0.06 MPa for 3 minutes again, and pouredonto a copper roll rotating at 1 m/sec to produce alloy strips. Theobtained alloy strips were retained at 1300° C. for 10 minutes, andrapidly quenched to room temperature, to thereby obtain an objectivealloy. The obtained alloy was subjected to the measurement of the oxygencontent and the determination of the PCT curve in the same way as inExamples 1-8. The results are shown in Table 3.

[0061] It is seen from Table 3 that even with the same alloycomposition, the alloy of Comparative Example 5 produced by aconventional method exhibited a higher oxygen content and a lowereffective hydrogen storage capacity, compared to those of the alloy ofthe present invention. TABLE 3 As-cast Alloy Heat-treated AlloyComposition of Effective Effective Alloy Starting Ratio of HydrogenRatio of Hydrogen Material O₂ Content BOC Phase Storage BOC PhaseStorage (atomic %) (wt %) (%) Capacity (%) (%) Capacity (%) Example 16Cr₄₉Ti₃₁V₁₅FeLa₄ 0.068 100 2.12 100 2.42 Comparative Cr₄₉Ti₃₁V₁₅FeLa₄0.180 100 1.85 100 2.15 Example 5

What is claimed is:
 1. A hydrogen storage alloy principally of abody-centered cubic crystal structure, and represented by thecompositional formula Cr_(a)Ti_(b)V_(c)Fe_(d)M_(e)X_(f), wherein Mstands for one or more elements selected from the group consisting ofAl, Mo, and W; X stands for one or more elements selected from the groupconsisting of La, Mm (misch metal), Ca, and Mg; a, b, c, d, e, and feach denotes a value in atomic percent, and 30≦a≦70, 20≦b≦50, 5≦c≦20,0<d≦10, 0≦e≦10, and 0≦f≦10, provided that a+b+c+d+e+f=100, wherein saidalloy comprises 0.005 to 0.150 wt % of O₂, and has hydrogenabsorption-desorption capability of not less than 2.2% of its weight ina temperature range from 0 to 100° C. and in a pressure range from 0.001to 10 MPa.
 2. The hydrogen storage alloy of claim 1, wherein e in thecompositional formula satisfies 0<e≦10.
 3. The hydrogen storage alloy ofclaim 1, wherein f in the compositional formula satisfies 0<f≦10.
 4. Thehydrogen storage alloy of claim 1, wherein e in the compositionalformula satisfies 0<e≦10, and f satisfies 0<f≦10.
 5. A method forproducing a hydrogen storage alloy of claim 1, comprising: melting step(a) of melting starting materials for a hydrogen storage alloy of claim1 to prepare an alloy melt; at least one deoxidizing step (b) selectedfrom the group consisting of deoxidizing step (b1) of blowing argon gasinto the alloy melt, deoxidizing step (b2) of retaining the alloy meltin vacuum of not higher than 0.1 Pa, and deoxidizing step (b3) of addingand retaining in the alloy melt one or more elements selected from thegroup consisting of La, Mm, Ca, and Mg; and casting step (c) ofsolidifying the alloy melt.
 6. The method of claim 5 further comprising,after step (c), step (d) of retaining a solidified alloy in atemperature range from 1150 to 1450° C. for 1 to 180 minutes, followedby cooling to 400° C. or lower at a cooling rate of not lower than 100°C./sec.
 7. The method of claim 5, wherein a melting temperature in saidmelting step (a) is not higher than 1700° C.
 8. The method of claim 5,wherein said starting materials in said melting step (a) comprise atleast one of a Fe—V alloy, a Cr—Ti alloy, a Cr—V alloy, or V metalcontaining Al prepared by the thermit process.